Potassium tetrachloridocuprate(II)

Potassium tetrachloridocuprate(II) is a salt with chemical formula K
2CuCl
4, also written as (K+
)2·[CuCl
4]2−.

Potassium tetrachloridocuprate(II)
Names
IUPAC name
Potassium tetrachloridocuprate(II)
Other names
Potassium tetrachlorocuprate, potassium copper(II) tetrachloride, dipotassium cupric chloride, mitscherlichite (dihydrate mineral)
Identifiers
  • InChI=1S/4ClH.Cu.2K/h4*1H;;;/q;;;;+2;2*+1/p-4
Properties
K2CuCl4 (anhydrous)
K2CuCl4·2H2O (dihydrate)
Molar mass 319.585 g/mol (dihydrate)
Appearance
greenish blue crystals (dihydrate)
Density 2.416 g/cm3 at 25 °C (dihydrate)[1]
Structure
(dihydrate:) Tetragonal.Point Group: 4/m 2/m 2/m (probable). Crystals, short prismatic along [001], or pyramidal {011}, minute; in stalactitic growths[2]
Hazards
Flash point Non-flammable
Related compounds
Other cations
 Cesium tetrachloridocuprate(II)
ammonium tetrachloridocuprate(II)
rubidium tetrachloridocuprate(II)
iron(II) tetrachloridocuprate(II)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

The compound is often found as the dihydrate K
2CuCl
4·2H
2O, which is a brilliant greenish blue crystalline solid.[1] This form also occurs naturally as the rare mineral mitscherlichite.[1][2]

The compound is also called potassium tetrachlorocuprate(II), dipotassium tetrachlorocuprate, potassium copper(II) tetrachloride, potassium cupric chloride and other similar names.

Dihydrate
Crystals of K
2CuCl
4·2H
2O (on 1mm grid)

Synthesis and natural occurrence

The dihydrate occurs rarely in nature near volcanic vents, e.g. in Mount Vesuvius, as the mineral mitscherlichite; which is named in honor of Eilhardt Mitscherlich (1794–1863), the German crystallographer and chemist who first synthesized the compound.[2] It was identified as pigment in some ancient artifacts.[3]

The dihydrate can be obtained by slow evaporation of a solution of potassium chloride (KCl) and copper(II) chloride (CuCl
2) in 2:1 molar ratio.[1][4]

Structure of K
2CuCl
4·2H
2O (mitscherlichite).

Crystal structure

The crystal structure of the dihydrate was partially determined in 1927 by Hendricks and Dickinson[1][4] and refined in 1934 by Chrobak.[5] The structure is tetragonal P42/mnm (136), Z=2, isostructural with ammonium tetrachoridocuprate(II) (NH
4)2CuCl
4·2H
2O and rubidium tetrachoridocuprate(II) Rb
2CuCl
4·2H
2O.[1][4][5][6][7] Each copper atom is immediately surrounded by two oxygen atoms and four chlorine atoms forming a hydrated tetrachloridocuprate(II) anion. Two of the chlorine atoms are about 0.75 angstroms further away than the other two. Each potassium atom is surrounded by four oxygen atoms, four copper atoms and eight chlorine atoms.[4]

Anhydrous

Synthesis

The anhydrous compound was reported in 1952 by C. M. Fontana and others.[8] In the next two decades others reported its heat of formation[9] and its structure.[6][7]

In the mid-1970s, however, its existence was questioned.[10][11] The phase diagram for the anhydrous system KCl/CuCl
2 shows potassium trichloridocuprate KCuCl
3 as a congruently-melting compound, but not K
2CuCl
4.[12] The dihydrate decomposes on heating above 93 °C to KCl, KCuCl
3 and water.[13][14]

The doubts were put to rest when successful dehydration was achieved by T. J. Nolan and others in 1975.[15][16]

See also

References
  1. Howard E. Swanson, Howard F. McMurdie, Marlene C. Morris, Eloise H. Evans, and Boris Paretzkin (1971) Potassium Copper Chloride Hydrate (mitscherlichite), in [ Standard X-ray Diffraction Powder Patterns] National Bureau of Standards, Monograph 25, Section 9; page 34.
  2. John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, and Monte C. Nichols (1997) Mitscherlichite, in Handbook of Mineralogy, Mineralogical Society of America. volume III.
  3. Riederer (1982)
  4. Sterling B. Hendricks and Roscoe G. Dickinson (1927). The crystal structures of ammonium, potassium and rubidium cupric chloride dihydrates Journal of the American Chemical Society, volume 49, issue 9, pages 2149–2162 doi:10.1021/ja01408a005
  5. Chrobak (1934)
  6. E. Joly (1971). C. R. Acad. Sci., Ser. C, volume 271, page 1302.
  7. I. V. Vasil'kova and G. M. Barvinok (1966). Chemical Abstracts, volume 65, page 9831.
  8. C. M. Fontana, E. Gorin, G. A. Kidder and C. S. Meredith (1952). Ind. Eng. Chem., volume 44, page 363
  9. S. A. Shchukarev, I. V. Vasil'kova and G. M. Barvinok (1965). Vestn. Leningr. Univ., Ser. Mat. Fiz. Khim., volume 20, page 145.
  10. H. T. Witteveen, D. L. Jongejan and W. Brandwijk (1974). Mater. Res. Bull., volume 9, page 345.
  11. D. W. Smith (1976), Chlorocuprates(II). Coordination Chemistry Reviews, volume 21, issues 2-3, pages 93-158. doi:10.1016/S0010-8545(00)80445-2
  12. M. P. Vorobei and O. V. Skiba (1970). Russ. J. Inorg. Chem., volume 15, page 725.
  13. R. Perret(1966). Bull. Soc. Chim. Fr., volume 1966, page 3190.
  14. H. Suga, M. Sorai, T. Yakamana and S. Seki (1965). Bull. Chem. Soc. Jpn., volume 38, page 1007.
  15. N. Fogel and T. J. Nolan (1975). Amer. Chem. Soc. Symp., section Inorg., page 121.
  16. Thomas J. Nolan, Harold Haralson, James L. McAdams and Norman Fogel (1977) Dehydration of potassium tetrachlorocuprate(II) dihydrate. Journal of the Chemical Society, Dalton Transactions, volume 1977, issue 17, pages 1608-1612 doi:10.1039/DT9770001608
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